This invention relates to an improvement of a process and apparatus for preparing a tubular article wherein a mandrel is used in extruding the tubular article from thermoplastic resin, and the form of the tubular article is adjusted by means of the mandrel.
It is already known that a tubular article can be prepared from thermoplastic resin by an extrusion process, wherein a die is mounted on the forward end of an extruder, said die being provided with an annular orifice, from which heated thermoplastic resin is extruded in the form of a tube. It is also known that the tubular article can be completed to a desired shape by what is called a mandrel process, wherein a rigid member which is called a mandrel is mounted on the forward end of the inner core of said die, said mandrel having an outer sectional shape corresponding to an inner shape of the article to be obtained the resin tube extruded from the die is advanced along outer surface of said mandrel. Further, it is also known that an extrusion efficiency can be increased by adopting the process, wherein cooling liquid is circulated in said mandrel so that the inner surface of the tubular article is cooled.
The inventor attempted to prepare a tube of polyethylene by using a mandrel according to the above known processes. Further, on carrying out the processes, the inventor attempted to enhance the extrusion efficiency by cooling the mandrel in order to rapidly cool the extruded polyethylene tube from its inner surface while advancing along the outer surface of the mandrel. The polyethylene tube could formed in the shaped just corresponding to the mandrel, however, the tube was not of good quality because the tube had rough portions on the surface. The rough portions are as shown in FIG. 1 the protuberances were scattered over the surface of the tube as if each of the protuberances were in the shape of a bowl turned upside down. The tube was therefore of remarkably less commercial value.
The inventor has prepared a die for use in preparing a resin tube, on inner core of which is mounted a mandrel, which has a groove and an aperture, said groove being located on the outer surface of the mandrel in the form of a ring surrounding the mandrel, said aperture being located in the groove and communicating with outside of the die through a pipe extending through the die. The die may be used in such a manner that the extruded resin tube is advanced in contact with the outer surface of the mandrel. This is done by discharging from the aperture the air contained between the mandrel and the resin tube.
The inventor has carried out the extrusion process by mounting the die on an extruder and by discharging the air to the exterior of the mandrel. This effects a close contact between the extruded polyethylene tube and the outer surface of a mandrel, and as the result the inventor has succeeded in solving the above drawbacks. In the conventional mandrel processes, attempts have been made to introduce the air into a gap between the mandrel and the extruded tube in order to avoid direct contact of the extruded tube with the mandrel so that the surface of the extruded tube may not be damaged by the mandrel. This, technique is opposite to the technique of the present invention. It is quite surprising to find that above attempt solves the drawbacks and thereby a polyethylene tube having an excellent quality can be obtained.